With ever-increasing demands for increased passenger safety and lightweight solutions in auto body structures, new steel grades for cold forming have been developed. Such steel grades, either as coated or uncoated steel strip, are cut or stamped into blanks which are cold stamped or cold hydro formed into a desired shape.
Steel grades used for cold forming have included micro alloyed high strength steels, dual phase steels, and multiphase steels such as retained austenite bearing TRIP steels and complex phase steels. However, when such steels exceed tensile strengths of 800 megapascals (MPa), formability of such materials is insufficient to form complex parts. In addition, martensitic steels with tensile strengths between 1000 and 2000 MPa have also been used/tested, however, the martensitic steels have been susceptible to delayed cracking problems due to hydrogen embrittlement.
In order to overcome the above-stated deficiencies, boron steels are currently used to produce complex shapes. For example, boron steel blanks are first heated and then hot formed or press hardened into a complex shaped component, followed by cooling of the component within the forming die. It is appreciated that such processing typically occurs on hot forming lines that include a blank destacking device, a furnace for heating the blanks to a desired elevated temperature, a hydraulic press or servo press, and additional components that guarantee process control with respect to time, speed, and temperature.
It is also appreciated that the hot forming process includes heating the blanks to a temperature above the austenite transformation temperature for a given particular alloy to be hot formed. In addition, the blanks are hot formed within the austenitic range where formability is high. Thereafter, the formed component is cooled using a cooling rate that is sufficient to prevent formation of ferrite, pearlite, and/or bainite, thereby resulting in a microstructure of martensite with only a small amount of retained austenite. Heretofor resulting parts/components have tensile strengths between 1000-2000 MPa, but the percent elongation to failure has been between 5-11%, with a decrease in ductility with increasing tensile strength close to a limit in product of tensile strength times percent elongation to failure of 11000 MPa·%. Therefore, an improved process and steel alloy combination that provides for elevated tensile strengths in the range of 1400-2400 MPa and tensile elongations of at least 10% and/or up to a TS of 1600 MPa or for tensile strength above 16000 MPa a product of tensile strength times percent elongation to failure of at least 16000 MPa·%. would be a desirable goal.
A coupled hot stamping and heat treating process has recently been suggested to achieve such a goal. Such a coupled system would require a process line that affords for quenching a part to a defined temperature above ambient temperature and then transferring the part inside the one and the same process line into a second furnace without significant temperature loss. However, such a process line would require investment into new press hardening lines and not utilize existing technology. In addition, significant effort and costs would be required to synchronize and achieve desired throughput required for cost efficient press hardening lines. As such, an improved process and steel alloy combination that can produce a desired strength and ductility combination using existing process line technology would be desirable.